Right channel hum and no signal

[Paul Birkeland]

I'd go ahead and replace it, but consider ordering some extras.  These don't fail on their own, so I'd be very, very carefully inspecting the little resistors on that 6AQ5 socket to be sure they are present and well soldered, as a missing one or one with a loose joint would allow for the voltage provided to the TL431 to go too high, and then the 431 would fail.

-PB

[Sp33ls]

https://www.mouser.com/ProductDetail/926-LM431BCZ-NOPB

^ This one works just fine, right?

Yeah, that's interesting... I wonder what did it. I'll buy several just in case I have a troubleshooting session ahead of me 

Hopefully it'll be as simple as replacing it and being back in audio nirvana.

Very much appreciate all the help, PB. I'm traveling this week, but I'll report back next weekend on my findings with the new LM431s.

[Paul Birkeland]

I believe we buy the ACZ version of the 431.

[Sp33ls]

Yeah, I looked at the datasheet and didn't see a noticeable difference. Do you know what differences there might be?

[Paul Birkeland]

I think there might be some slight noise differences.

[Sp33ls]

Okay, I could see that. The only real difference I saw between the two was that the "initial accuracy" was 2% for ACZ and 1% for BCZ. 

I sent Mouser an email to see if they could swap it to ACZ just to be certain there won't be any issues. I also picked up a few extra transistors and diodes just to have as backup.

Does the LM431 protect the capacitors from hitting their voltage limits?

Thanks again!

[Paul Birkeland]

No, the LM431 regulates the high voltage to 150V.

[Sp33ls]

So, I swapped out the LM431 and plugged everything (including all the tubes) back in to see.

Still having the issue. But, I've noticed something interesting.

When I first turn on the amp (cold start), the voltages actually appear to be closer to normal (150V & like 2.5V), but then after several seconds, they both just take a sudden dip. What could cause behavior like that... a capacitor?

[Paul Birkeland]

Pull the 6AQ5s and look at your Kreg voltages again, as well as the "X" voltage.

[Sp33ls]

With or without the white lead from the 6AQ5 plugged in to Kreg?

[Paul Birkeland]

The white lead can be connected.

[Sp33ls]

KregA: 0.4
KregB: 1.38

60 VDC & 1 VDC on A side.

214 & 2.18VDC on B side.

[Paul Birkeland]

How about OA on the high current board on the bad side?

[Sp33ls]

It was around 50VDC... oscillating a bit upwards.

[Paul Birkeland]

With the 6AQ5 pulled out? (And the 6080 should be pulled out, but 12AU7 in) This is a bit of a tough one to debug since you said the amp was previously working.  Low voltage on OA on the C4S board tends to point to a miswire or short downstream of that board.